The present invention relates to the control of substrate temperature. More particularly, the present invention relates to an apparatus and method for controlling temperature uniformity of a semiconductor wafer and similar workpiece during processing.
In typical processes for manufacturing semiconductor devices, a thin, flat disc of semiconductor material referred to as a xe2x80x9cwaferxe2x80x9d is subjected to processes such as chemical vapor deposition, etching, coating, photographic patterning and other processes which form the desired features on the wafer. In many of these processes, it is important to maintain the entire wafer at a uniform temperature. It is particularly important to maintain a wafer at uniform temperature during so-called xe2x80x9cpost-exposure bakexe2x80x9d of a photoresist. In certain semiconductor processing procedures, a photoresist on a wafer is exposed to illumination in a pattern, and then baked to develop the resist. Depending upon the type of resist used, the areas of the resist which were illuminated may be either more readily cured or less readily cured. The post-exposure baking step after illumination helps create the pattern of cured and uncured areas.
One approach which has been used heretofore to maintain a uniform temperature in a semiconductor wafer is to provide a massive block of a metal having high thermal conductivity in contact with the wafer. The block may be heated, as by electrical heating elements. The thermal conductivity of the block helps to limit temperature differences between locations on the wafer. However, systems of this nature still suffer from significant temperature non-uniformity. A further approach is a multiplicity of individual controlled areas.
The present invention provides a simple yet effective device, which can be used to maintain the temperature of semiconductor wafers and other workpieces.
One aspect of the invention provides a workpiece holder. A workpiece holder according to this aspect of the invention includes a wall structure defining a substantially closed chamber. The wall structure includes a workpiece-engaging wall in thermal communication with the chamber. The workpiece holder also includes a condenser that communicates with the chamber and a heater that applies heat to the chamber. For operation, the heater applies heat to a fluid inside the chamber so that the fluid vaporizes within the chamber. When the fluid vaporizes the condenser abstracts the heat from the fluid and condenses it. The fluid throughout the chamber will be maintained substantially at the same temperature. This temperature corresponds to the vaporization temperature of the fluid at a pressure prevailing within the chamber and condenser. Although the present invention is not limited by any theory of operation, it is believed that the condenser and heater cause continual circulation of vapor through the interior of the chamber, which helps to maintain temperature uniformity.
The workpiece holder may include a feedback control system connected to one or more temperature sensors. The feedback control system is also desirably connected to the heater, condenser or both. The feedback control system operates to control the heater, the condenser or both responsive to signals from the temperature sensors so as to maintain a predetermined set point temperature within the chamber.
Another aspect of the invention includes a method of holding the workpiece at a uniform temperature. The method according to this aspect of the invention desirably includes maintaining the workpiece on a holding surface in heat transfer relation with the interior of a chamber; vaporizing a liquid disposed within the chamber to form vapor by supplying heat to the interior of the chamber; and condensing the vapor in a condenser that communicates with the chamber to ensure that some liquid and some vapor are present in the chamber and the vapor continually flows within the chamber and condenser.
Yet another aspect of the invention includes a method of making the workpiece holder. This method includes forming a wall structure containing a workpiece-engaging wall that is defined substantially as a closed chamber. This wall structure is subjected to a preselected pressure differential between the interior and exterior of the chamber corresponding to the pressure differential expected on the wall structure during use. As a result of the pressure differential exerted the workpiece-engaging wall will deform. Most preferably, the workpiece-engaging wall is machined to a preselected shape while it is in its deformed condition. Thus, during use of the workpiece holder, while the workpiece-engaging wall is under the pressure differential, it will have the preselected shape formed by the machining process.
Additional features and advantages of the invention will be set forth in the description which follows. It is to be understood that both the foregoing general description and the following detailed description are exemplary and are intended to provide further explanation of the invention as claimed.